Billionaire entrepreneur Elon Musk sees a future in which super batteries change the world , making solar power available at night and turning homes into tiny utilities. Kellie Haynes, an event planner in Sacramento, Calif., is one of the few Americans who already lives in that world. She says she loves the benefits but didn’t have to cover all the costs. Whether people are willing to pay thousands of dollars apiece to join her remains one of the biggest questions hanging over Mr. Musk’s Tesla Motors Inc. TSLA 1.62 % and other companies jumping into the budding business of electricity-storage batteries. Ms. Haynes lives in a year-old house with solar panels and a battery system that cost her nothing—the $25,000 system designed by San Francisco company Sunverge Energy Inc. was covered by government subsidies and utility incentives , according to developer Pacific Housing Inc. that built her 34-house neighborhood […]